In the future ever higher storage capacities and data rates will be required for computers and for devices in consumer electronics as well as in the multimedia field. In the case of optical memories, a higher storage capacity can be achieved by reducing the information memory locations and the light spot which scans the information layer. For this purpose, it would be necessary, for example, to use light having a shorter wavelength or an objective lens having a higher numerical aperture. Since laser diodes having an arbitrarily short wavelength are not, however, available, these two methods can also be combined. The information memory locations on the information carrier, which are designated as pits in the case of the CD, can then be shorter, on the one hand, and the distance to the neighbouring track can be reduced, on the other hand, without undesired crosstalk thereby occurring. Furthermore, optical information carriers such as, for example, read-only systems, rewritable disk systems, phase-change disks, magneto-optical disks or optical information carriers yet to be expected differ from one another in the method of information storage, which methods frequently also permit different storage densities. However, it is not directly possible by means of a system for reading out from information carriers having a relatively high storage capacity to read a disk having a lower density, that is to say larger pits and a larger track pitch, or to write information on it.
On the one hand, the pits or information memory locations representing data are too large, in comparison with the scanning light spot with the result that intensity modulation in the reflected light occurs only at the initial and final edges of the pits and, on the other hand, in the case of the use of the so-called three-beam track following method, which is generally used for track guidance, a changed adjustment of the secondary or auxiliary beams provided for the track guidance is necessary because of the different track pitch.